This invention relates to a process for forming polymers of .alpha.-methylstyrene and acrylonitrile and more particularly to polymerization rate improvements in such a process.
Polymers of chemically combined .alpha.-methylstyrene (AMS) and acrylonitrile (AN) containing about 70 weight % polymerized AMS are well known as possessing significantly higher heat distortion temperature (on the order of 17.degree. C.) than a styrene (S)-acrylonitrile copolymer (SAN) of comparable styrene content. AN provides chemical resistance and toughness in the polymer. As typically disclosed in examples of UK 1,390,973, AMS/AN copolymers can be blended with impact resistant copolymers of styrene and acrylonitrile grafted on a diene rubber (ABS polymers) to improve the heat resistance of the latter. In this respect, AMS/AN copolymers at about a 70/30 weight ratio are chemically compatible with the ABS polymers and therefore conventionally used in such blends. Furthermore, a 70/30 AMS/AN weight ratio essentially represents the azeotropic composition, by which is meant the mixture containing a weight ratio of monomers which when polymerized provide the same ratio of chemically combined monomers in the polymer. As further known, it is desirable, when possible, to use the azeotropic composition during a polymerization since a different off-azeotrope ratio of monomers in a closed, batch polymerization system will produce a polymer composition corresponding to the proportion of monomers in the polymerizing medium. In the latter instance, the polymer undesirably varies widely in compositional distribution because of the changing composition of the monomers combining to form the polymer during the course of the polymerization. Thus, it seems conventional in the prior art to use the azeotropic composition of monomers which produce a polymer having the same ratio of chemically combined polymerized monomers.
As further known, (note U.S. Pat. No. 4,169,195) AMS polymerizes slower than styrene and this has typically been remedied in the prior art (note U.S. Pat. No. 4,618,663) by using special difunctional initiators to increase polymerization rate. Though this approach can be used, it is not without drawbacks; for example, such initiators are chemically complex and not inexpensive and, though used in minor amount, residues in the polymer can adversely affect the balance of performance properties. Moreover such special initiators produce only a marginal improvement in rate.
It would be desirable to have another way of increasing the notoriously slow polymerization rate of AMS in forming polymers with AN.